Head Gasket leak on 99 VW passat?

I am looking into buying a used 1999 Volkswagen passat and I have asked about the cars maintenance. The owner did mention that the water pump was replaced but at the moment there is a leaky valve seal on the head gasket. I am not familiar with car engines what so ever. Is this a major issue? Or will it lead to being a major issue? If so, is it an expensive thing to fix/replace??